The present invention relates to a worm differential gear mechanism for using a worm gear connection.
An example of prior art related to the present invention is shown in the Japanese Utility Model Laid Open Publication No. 2-117457. This has a differential case, a pair of worm gears and a plurality of element gears. The differential case is rotated by an engine via a ring gear which is provided in an outside circumference of a bottom side of differential case. The differential case retains two axle shafts in a rotatable condition, one of the top side where no ring gear is provided and the other on the bottom side where a ring gear is provided. A pair of worm gears are provided in a serial arrangement and are rotatable in the differential case, and are each connected with the different axle shafts. The element gears are each composed of a worm wheel, and spur gears provided on both sides of the worm gear. The element gears are arranged in one set of two element gears and a plurality of sets are provided at openings for attaching element gears which are formed on a side face of the differential case. A plurality of sets of element gears are provided around the worm gears so as to intersect orthogonally with the worm gears. These element gears are retained in the differential case and are rotatable. Each set of element gears of their spur gears engage with each other, and their worm wheels engage with each of the worm gears.
In the above-mentioned worm differential gear mechanism, as shown in FIG. 6, journal pin insertion ports on the top and bottom sides of the differential case are formed in line along an axial direction of the differential case in vicinity of the same opening for attaching element gears which is located on a side face of the differential case between openings for attaching element gears. And, each of a set of element gears is rotatably retained by the differential case by means of journal pins which are inserted into journal pin insertion ports of the top and bottom sides.
FIG. 6 is a development of the differential case showing journal pin insertion ports and an inserted condition of journal pins in the above-mentioned worm differential gear mechanism. In FIG. 6, a reference numeral 1 is the differential case, reference numerals 2a, 2b and 2c are openings for attaching element gears, reference numerals 3a, 3b, 3c, 3d, 3e and 3f are element gears, reference numerals 4a, 4b, 4c, 4d, 4e and 4f are journal pin insertion ports, and reference numerals 5a, 5b, 5c, 5d, 5e and 5f are journal pins. The journal pin insertion port 4a on the top side and the journal pin insertion port 4b on the bottom side are formed in line along an axial direction of the differential case 1 in vicinity of the opening 2a for attaching element gears. And, the journal pin 5a on the top side and the journal pin 5b on the bottom side retain a set of element gears 3a and 3b rotatably to the differential case 1. Other journal pin insertion ports (4c, 4d) and (4e, 4f) and journal pins (5c, 5d) and (5e, 5f) also constitute the same composition in regard to the openings 2b and 2c for attaching element gears and the element gears (3c, 3d) and (3e, 3f).
In the mean time, in case of this type of worm differential gear mechanism, and taking a side face of the differential case 1 between the openings 2a and 2b for attaching element gears for example, a maximum load works on a portion A1 on the top side of the differential case 1, which is located in vicinity of one opening 2a for attaching element gears, during driving when a drive system drives the differential case 1 rotationally towards a forward direction of a car via the ring gear, and, during coasting such as engine braking when the axle shafts drive the differential case 1 rotationally towards a forward direction of the car, the maximum load works on a portion B1 on the bottom side of the differential case 1, which is located in vicinity of another opening 2b for attaching element gears. Likewise, between the openings 2a and 2c for attaching element gears, and between the openings 2c and 2b, maximum loads work on top side portions A2 and A3 during driving, and on bottom side portions B2 and B3 during coasting. During reverse driving and coasting, these loads occur reversely. During backward driving, maximum loads work on bottom side portions B1, B2 and B3, and in the backward coasting, the maximum loads work on top side portions A1, A2 and A3.
As it is clear from FIG. 6, in case of a conventional worm differential gear mechanism, maximum loads work on portions which are thinned because of insertion ports 4b-4f because journal pin insertion ports 4b, 4d and 4f on the bottom side are located in vicinity of or overlapped with portions B1, B2 and B3 where maximum loads work. Because of this, in case of the ordinary worm differential gear mechanism, big anxiety is left in regard to durability of the differential case. As a method to improve this, enlargement of an outside diameter of the differential case can be considered. However, if the outside diameter is enlarged, it will increase the size of the differential case.